Problem: The scale on a map is 5cm : 3km. If the distance between two cities is 9km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 5cm on the map corresponds to an actual distance of 3km. An actual distance of 9km is the same as 3 $\cdot$ 3km. The distance between the two cities on the map is 3 $\cdot$ 5cm, or 15cm.